AR-15 Extractor Improvements:
In recent years some shooters have come to realize that many of their AR15 malfunctions can be traced to weak extraction. To over come this weak extraction some clever folks came up with the idea of placing up to two O-rings over the extractor spring. This configuration has the O-rings serving as the primary spring with the factory extractor spring serving as a guide for the o-rings as well as a secondary spring.
If someone chooses to use this method the proper O-rings must be utilized. The O-ring should be selected for not only its dimensions but also its physical properties. You don't want the O-rings to disintegrate and fall apart from firing and cleaning the weapon. The correct O-rings can be purchased quickly and easily from online industrial supplier Mcmaster Carr. The Mcmaster part# is 1201T16 (Same as 1201T111). The cost is about $5 for 50!. If you wish to purchase them elsewhere the O-ring specification is as follows MIL-Spec Viton® Fluoroelastomer O-Ring AS568A Dash Number 006.
The MGI D-Fender "D-Ring" is the next evolutionary step for the O-ring method listed above. MGI took the O-ring concept and made a ring in the shape of the Extractor's radiused end. The ring forms a "D" shape. The D-ring is placed over the extractor spring in the same manner as the o-rings with the orientation of the "D" matching that of the profile of the extractor.
MGI claims a 4x increase of extractor force. I have no reason to doubt their claim.
The D-Fender is available from Brownells here
Bravo Company's BCM Extractor Spring Upgrade Kit takes the O-ring method utilize one O-ring around the extractor spring and adds a rubber buffer inside the extractor spring. Bravo Company states that in most AR15's the external O-ring will not be needed and that most likely is accurate. DPMS ships their AR15 bolts with a similar system to the BCM less the external O-ring.
The Buffer Technologies Extractor Upgrade Kit is identical to the BCM Kit above. Buffer Technologies states that their kit contains an extra power extractor spring. I could not confirm this of the BCM kit
These kits cost about $13 at Brownells
The extractor buffers from the BMC and Buffer Technologies kits are available as separate parts from Brownells for about $14 for a quantity of 10. I like this idea as the buffer is captive in the spring and as I mentioned earlier DPMS ships their AR's with a buffer in the extractor spring. In fact Colt offers a replacement extractor spring with a buffer here. It also appears Brownells has several variations.
 
Replacements springs - this is quite simply replacing a stock extractor spring with one of much higher force. The springs are meant to be used without any buffers or O-rings.
Below is a list of sources for the heavy duty extractor springs.
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